The present invention relates to an underground facility for the storage of liquids and, more particularly, to a fuel storage tank within a segmentalized precast concrete vault, the assembly having a highly efficient fuel storage capacity to overall displacement volume ratio.
Underground storage tanks are frequently used for the storage of various liquids, such as gasoline, fuel oil, diesel oil, toxic fluids or various chemicals. These underground storage tanks are used in retail automobile service stations, truck and bus depots, for various industrial and commercial facilities, and occasionally, for homes and consumer purposes. The storage tanks are generally tubular, consist of a welded construction of sheet steel of sufficient gauge, and have a capacity ranging from 500-50,000 gallons.
For some time the Environmental Protection Agency (EPA) has recognized that the United States faces a costly underground storage and pollution problem. The old methods of storing liquids, chemicals and hazardous materials are not acceptable. Gasoline stations, for example, previously used single-wall direct bury tanks for their on-site gasoline storage. After a relatively short time, these underground storage tanks leak--contaminating the ground water in the surrounding land and endangering the public. Detecting these leaks is difficult and usually only occurs after gasoline has been leaking for a considerable time. Replacing or repairing these underground tanks is very expensive and time-consuming. More recently, some tanks have been made of fiberglass, but such tanks are subject to cracking or other problems which cause these tanks to leak also.
As existing underground tanks are, on average, 25 years old, and as the risk of leaks increases substantially after 12 years, it is currently estimated that up to about 20% of underground storage tanks are probably leaking. In view of concerns regarding environmental problems associated with leaking underground storage tanks, some years ago the EPA adopted regulations requiring regular leakage testing of underground storage tanks and the carrying of insurance policies or providing evidence of financial responsibility to cover the cost of any required environmental clean-up. Complying with these regulations has significantly added to the cost and responsibility of owning and operating underground storage tanks. In many cases, the required insurance, if available at all, is so expensive that independent gas station owners cannot afford it.
The EPA's regulations do not apply, however, when the storage tanks themselves are not directly buried but are placed within a structure or vault where they can be inspected and where any leakage can be contained. Hence, placing storage tanks within a structure, either above ground or underground, is a way to both gain exclusion from EPA regulations and prevent environmental problems. In addition, contamination concerns of financiers and future land owners, as well as the potentially crippling expense of contamination clean-up, are alleviated with the use of a storage tank within an enclosing structure.
To qualify for the EPA underground storage tank regulation exclusion, the tank must be situated upon or above the surface of the floor of an underground area such that inspection for leaks is possible. However, an inspector must first gain access to an underground vault to visually inspect the exterior of an enclosed tank. In order to comply with OSHA (Occupational Safety and Health Administration) access regulations, there is a minimum access ladder area width requirement of 30 inches and at least an 18 inches wide confined access space between the sides of the tank and vault is required.
U.S. Pat. Nos. 4,638,920 issued to Goodhues, Jr., 4,961,293 to House, et al. and 5,037,239 to Olsen, et al. disclose underground concrete vault structures designed to enclose hazardous liquid storage tanks. These patents show tubular tanks within generally rectangular parallelepiped vaults where a certain amount of space or clearance is left around the vaults to provide for access and visual inspection.
In urban areas, it is common for service stations and convenience store facilities to be located on an expensive, busy intersection site where rights of way and utility easements are enlarged as the area develops. Frequently, these public acquisitions result in tank encroachments that must be resolved. Typically, an aging storage tank which is positioned partially in a public easement space must be replaced due to leaking. Once the owner has removed the existing tank and/or vault, the city or municipality often refuses to allow the space within the public easement to be used when situating a new tank.
Unfortunately, this problem cannot always be overcome by digging a deeper hole in the ground to install a taller tubular vault. Depending on the local geology, the water table may be from 2 feet deep in coastal areas to around 15 feet deep inland. Safety regulations become stiffer when digging below the water table due to the extra shoring needed and larger excavation machines to reach deeper below ground. Thus, digging deeper becomes more expensive and is often not a viable alternative.
Prior underground vaulted tanks have exclusively utilized horizontal tubular internal tanks, common in the retail gasoline industry. Tubular tanks are considerably cheaper to manufacture than other configurations. It is well-known that a round internal tank is more economical than other shaped tanks to contain liquids because of the reduced stresses inherent in the design. However, the concrete vaults surrounding the internal tank are typically constructed with rectangular cross sections due to the ease of digging similarly-shaped holes in the ground and forming concrete in flat walls rather than round walls, as well as the need to provide a flat surface as a platform for inspection access.
Other drawbacks are also associated with the installation of existing underground vaulted tanks. Assuming the assembly of an outer square cross-section concrete vault pre-cast in pieces, the difficulties associated with transportation of such large pieces becomes significant. There are restrictions on trucking capacity, not only for traffic safety, but also because the roads have a maximum load bearing strength. Furthermore, the cranes used to lift the pieces onto waiting trucks and then to unload the concrete sections have a maximum tonnage lift capacity. There is typically a maximum crane size which is practical for on-site installation, thus limiting the possible site access.
One example of an existing vaulted tank is a 10,000 gallon storage capacity unit manufactured and sold by Secondary Containment Vaults (SCV) of San Antonio, Tex. under the trademark SUREVAULT. The vault portion consists of six prestressed, precast units: a bottom unit, a collar unit and a cover comprised of four flat panels. The storage tank is a cylinder having a length of approximately 28 feet and a diameter of 8 feet. The outer dimensions of the rectangular vault are 31'-8" by 12'-0" by 11'-8.5". The largest component is the bottom unit which forms a cradle for the tank and weighs 79,300 lbs. Disadvantageously, most states require special trucking permits for loads greater than 50,000 lbs and, in order to transport such a large piece, an escort vehicle may be necessary. Furthermore, a crane having an 140 ton capacity would be needed to lift the largest piece safely at a 25' reach, and such a crane typically rents for a steep hourly rate as compared to a smaller sized crane having a 65 ton capacity. For instance, in one region, crane rental rates for a 140 ton crane are 175% of the cost of a 65 ton crane.
Rectangular parallelepiped tanks encased in concrete are presently used for above-ground tank installation. The primary purpose of the concrete encasement of an above-ground tank is to provide fire protection. However, the concrete encasement also provides secondary fuel containment. Typically, the clearance between the concrete and the inner tank varies from approximately 1/4 inch to 2 inches to serve as a space for allowing free transport of any primary tank leakage to a lower point where the leakage can be readily sensed by various means of leak detection. These designs neither permit, nor do they meet EPA standards for visual inspection of, or OSHA standards for physical access, to the tank.
Despite the fact that rectangular tanks cost approximately twice as much to build as a tubular tank, rectangular internal tanks are used in rectangular above-ground vaults because the exterior concrete that encases the interior tank can more easily be monolithically cast, thereby eliminating the need for joints in the concrete which present sealing, and therefore potential fire, problems. Examples of such above-ground storage tanks are shown in U.S. Pat. Nos. 4,931,235 and 4,934,122. This type of fire risk is not significant in the underground tank environment.
There is currently a need for an improved underground storage tank within an access vault which addresses the limitations of the prior art.